The invention is related to a hydraulic anti-locking brake system of a type having a master brake cylinder, at least one wheel brake, a pump, a low-pressure accumulator and a high-pressure accumulator, as well as inlet and outlet valves for the control of the brake pressure and an isolating valve for shutting off the master brake cylinder. The isolating valve and the inlet valve are incorporated in series in the brake line which links the master brake cylinder to the wheel brake. The outlet valve is inserted in the return line which links the wheel brake to the low-pressure accumulator. The pump delivers fluid from the low-pressure accumulator into the high-pressure accumulator. A non-return valve is in the brake line between the isolating valve and the inlet valve.
A brake unit of this kind is known from the German Patent Application published without examination, No. 3,603,533. In that brake unit, the master brake cylinder is isolated from the brake circuit during a brake pressure control action. Hydraulic fluid is withdrawn from the wheel brake cylinders in order to reduce the pressure in the wheel brakes. The hydraulic fluid is conveyed by a pump into a high-pressure accumulator. In order to bring about a renewed pressure build-up, a relevant inlet valve is opened so that hydraulic fluid is conveyed back from the accumulator into the wheel brakes. The maximum receiving capacity of the accumulator is such that the accumulator is capable of holding the total volume of hydraulic fluid which is contained in the wheel brake cylinders. Such a requirement may, for instance, come about if and when the friction conditions between the tires and the road surface should suddenly change from high to low friction coefficients.
Such an accumulator requires a considerable mounting space. It is, therefore, the object of the invention to provide a brake unit with a high-pressure low volume accumulator which allows a large volume flow to be applied in the brake system.